The present invention relates to a laser having a frequency conversion element and, in particular, an ultraviolet laser using a non-linear optical element.
A conventional ultraviolet laser utilizes a non-linear optical element such as a second-harmonic generation (SHG) element, which in response to a visible laser light emits ultraviolet rays by wavelength conversion. An example of a conventional ultraviolet laser which uses an argon laser tube as a visible laser light source and .beta. barium borate crystal (.beta.-BaB.sub.2 O.sub.4 ; referred to as BBO crystal, hereinafter) as an SHG element is disclosed in "High Efficiency Frequency Multiplication of Continuous Wave Ion-Laser", Applied Physics, Vol. 61, No. 9 (1992), pp. 931-934. In the disclosed ultraviolet laser which is shown schematically in FIG. 1, an argon laser tube 101, a condenser lens 104 and an SHG element 105 disposed between a pair of cylindrical lenses 106 are arranged in that order between a reflecting mirror 102 and an output mirror 103 to form an optical resonator. A fundamental wave having wavelength of 514 nm from the argon laser tube 101 is converted into a second-harmonic wave (wavelength of 257 nm) by passing through the SHG element 105 of the optical resonator and ultraviolet light is emitted from the output mirror 103. Since wavelength conversion efficiency of the SHG element is proportional to a square of laser field density, it is improved by condensing laser light by the condenser lens 104. In this laser, since tolerance width of angular phase matching (incident laser light deviation at which wavelength conversion efficiency becomes a half) of the BBO crystal 105 which is the SHG element in a first phase matching in which an optical axis of the BBO crystal 105 is in a horizontal direction in the drawing sheet (polarizing direction of argon laser) is larger than that in a direction perpendicular to the horizontal direction, the laser beam is enhanced by the cylindrical lenses 106 to thereby improve the conversion efficiency.
In this prior art, however, there is a tendency of the actual incident angle of the fundamental wave to deviate with respect to the phase matching angle of the SHG element due to vibration, mechanical shock and/or environmental temperature variation during its use, since the SHG element is arranged between the cylindrical lenses and such deviation is difficult to be regulated. For these reasons, the wavelength conversion efficiency thereof is degraded. Particularly, since wavelength conversion efficiency of the BBO crystal which is a typical SHG element for ultraviolet light is reduced by half upon incident angle deviation of only 0.1 degree, an oscillation of such conventional laser as shown in FIG. 1 which includes many optical parts to be optically regulated may easily go undetected.
Further, in the example shown in FIG. 1, reflection loss between the cylindrical lenses and the SHG element is large and highly efficient wavelength conversion is difficult.